(a) Field of the Invention
The present invention relates to a waveguide-type variable-sensitivity semiconductor photodetector and, more particularly, to a semiconductor photodetector suitable for use in an optical communication system, which is capable of generating an output signal having an excellent linearity to a large-amplitude input optical signal, thereby achieving a wide dynamic range for the input signal.
(b) Description of the Related Art
It is important to provide a photodetector capable of obtaining a constant amount of photocurrent without depending on the magnitude of the incident light, that is, generating an output signal having an excellent linearity to a large-amplitude input optical signal. Such a photodetector can achieve low fabrication costs for optical modules generally used in public optical communication systems. A waveguide-type variable-sensitivity semiconductor photodetector (referred to as simply variable-sensitivity photodetector, hereinafter) is proposed which is capable of theoretically obtaining the constant amount of photocurrent by optical absorption.
The variable-sensitivity photodetector of the type as mentioned above generally comprises a waveguide photodetector area and a variable-sensitivity waveguide area disposed as the input stage of the waveguide photodetector area. It is proposed that the variable-sensitivity waveguide area be implemented by an optical attenuator for attenuating the magnitude of input optical signals. The optical attenuator has a function for increasing the optical absorption in the optical waveguide area depending on the reverse-bias voltage to thereby reduce the optical input received in the waveguide photodetector area.
Referring to FIG. 1, a conventional variable-sensitivity photodetector 40 has a waveguide photodetector area 12, and an optical attenuation area or variable-sensitivity waveguide area 14 disposed as the input stage of the waveguide photodetector area 12 and having a function of reducing the optical input received in the photodetector area 12. The variable-sensitivity photodetector 40 has a layer structure wherein an InP lower optical confinement layer (or cladding layer) 18, an optical waveguide layer 20, an InP upper cladding layer 22, and a GaInAs contact layer 24 are consecutively formed on an n-InP substrate 16. The photodetector area 12 and the optical attenuation area 14 are electrically separated by a slit-like separation groove 26 for dividing the contact layer 24 into two parts, on which separate top p-electrodes 28a and 28b are formed. The optical waveguide layer 20 including layers 20a and 20b in both the areas 12 and 14 has different thickness between the layers 20a and 20b. A bottom n-electrode 30 formed on the bottom of the substrate 16 is common to both the areas 12 and 14 and is grounded.
The optical waveguide layer 20b in the optical attenuation area 14 is made of a semiconductor material having a bandgap energy larger than the optical waveguide layer 20a in the photodetector area 12 has, and is formed as a transparent waveguide having a small optical absorption rate, which can be practically neglected, against the operative wavelength for the photodetector area 12.
In a general heterojunction structure, wherein two semiconductor layers made of different materials are in contact with each other, the conduction band and the valence band involve distortions therein to have an equal Fermi-level in the bandgap diagram, which causes a band discontinuity at the heterojunction interface therebetween. The band discontinuity, especially in the p-type valence band, involves large spikes, which accumulate or pile-up halls generated by excitation due to optical absorption at the heterojunction interface in a photodetector or optical modulator. This phenomenon is reported as "hall pile-up effect" in IEEEJ and Quantum-Electron 31 pp.261-268, 1995 by D. Meglio et al, for example.
Hall pile-up caused to lower the electric field applied to the optical waveguide layer in the variable-sensitivity waveguide area, degrading the linearity of the output optical signal to the input optical signal. This results from the fact that a larger amount of incident light reduces the absorption coefficient of the photodetector.
The conventional variable-sensitivity photodetector, such as an integrated waveguide-type attenuator, generally has a heterojunction structure between the optical waveguide layer and the cladding layer. As a result, the conventional variable-sensitivity photodetector is liable to the adverse effect by the pile-up of holes in the variable-sensitivity waveguide area, which renders difficult to achieve the linearity of the output optical signal to the input optical signal. Thus, the output characteristics of the variable-sensitivity photodetector as to distortion characteristics are deteriorated to make it difficult to achieve low cost optical modules for use in a public optical communication system.